The present invention relates generally to determining channel estimates for a MIMO communication system, and more particularly to determining such channel estimates in slow and fast fading conditions.
Release 7 of the UMTS standard uses a common 256-chip spreading sequence to generate an extended 512-chip spreading sequence for pilot signals generated by respective transmitters in a Multiple-Input, Multiple-Output (MIMO) communication system. To distinguish between the transmitters, the polarity of the common spreading sequence in the extended spreading sequence is alternated every 256 chips for one transmitter relative to the other transmitter. This produces two mutually orthogonal 512-chip spreading sequences for the pilot signals.
The receiver may despread the received signal using the extended 512-chip spreading sequences to determine the channel estimates for each pilot signal. This process provides one channel estimate for every 512 chips of each pilot signal. For a pilot signal having ten symbol periods per slot, where each symbol period contains 256 chips, this process provides five channel estimates per slot for each pilot signal. Conventional systems average the five channel estimates over the slot to gain a slot-specific channel estimate with reduced noise and/or interference.
The slot-specific channel estimates resulting from the above-described process typically have sufficient accuracy for slow fading conditions, but not for fast fading conditions. Thus, there remains a need for determining accurate channel estimates under fast fading conditions in MIMO communication systems.